The protonation of methyl yellow (MY) leads to a tautomeric equilibrium involving the azo and hydrazone species, where the latter is predominant. Electronic and Raman spectroscopic data show that when MY in acidic medium is included in cyclodextrins, there is an inversion in the relative ratio of tautomers, in which the azo species become the major species. This indicates that the azo bond is included in cyclodextrin precluding its protonation. The understanding of the protonation, tautomeric and inclusion equilibria of these systems plays an important role in the designing of cyclodextrin based molecular machines controlled by light. (C) 2011 Elsevier B. V. All rights reserved.